Listen to brigtness updates to update brightness summary.

- add content observer for brightness relates changes, and update the
brightness preference summary when changes occur.
- also check for VR mode and auto brightness mode when reading the
current brightness value.

Change-Id: I611ec77174ab45315ccbee2952bdbc2c9a9cd954
Fix: 37227609
Test: make RunSettingsRoboTests

src/com/android/settings/DisplaySettings.java

@@ -27,6 +27,7 @@ import com.android.settings.core.lifecycle.Lifecycle;
 import com.android.settings.dashboard.DashboardFragment;
 import com.android.settings.display.AutoBrightnessPreferenceController;
 import com.android.settings.display.AutoRotatePreferenceController;
+import com.android.settings.display.BrightnessLevelPreferenceController;
 import com.android.settings.display.CameraGesturePreferenceController;
 import com.android.settings.display.DozePreferenceController;
 import com.android.settings.display.FontSizePreferenceController;
@@ -106,6 +107,7 @@ public class DisplaySettings extends DashboardFragment {
         controllers.add(new VrDisplayPreferenceController(context));
         controllers.add(new WallpaperPreferenceController(context));
         controllers.add(new ThemePreferenceController(context));
+        controllers.add(new BrightnessLevelPreferenceController(context, lifecycle));
         return controllers;
     }